Module java.base

Package java.util.function

Functional interfaces provide target types for lambda expressions and method references. Each functional interface has a single abstract method, called the functional method for that functional interface, to which the lambda expression's parameter and return types are matched or adapted. Functional interfaces can provide a target type in multiple contexts, such as assignment context, method invocation, or cast context: 

     // Assignment context
     Predicate<String> p = String::isEmpty;

     // Method invocation context
     stream.filter(e -> e.getSize() > 10)...

     // Cast context
     stream.map((ToIntFunction) e -> e.getSize())...

The interfaces in this package are general purpose functional interfaces used by the JDK, and are available to be used by user code as well. While they do not identify a complete set of function shapes to which lambda expressions might be adapted, they provide enough to cover common requirements. Other functional interfaces provided for specific purposes, such as FileFilter, are defined in the packages where they are used.

The interfaces in this package are annotated with FunctionalInterface. This annotation is not a requirement for the compiler to recognize an interface as a functional interface, but merely an aid to capture design intent and enlist the help of the compiler in identifying accidental violations of design intent.

Functional interfaces often represent abstract concepts like functions, actions, or predicates. In documenting functional interfaces, or referring to variables typed as functional interfaces, it is common to refer directly to those abstract concepts, for example using "this function" instead of "the function represented by this object". When an API method is said to accept or return a functional interface in this manner, such as "applies the provided function to...", this is understood to mean a non-null reference to an object implementing the appropriate functional interface, unless potential nullity is explicitly specified.

The functional interfaces in this package follow an extensible naming convention, as follows:

  • There are several basic function shapes, including Function (unary function from T to R), Consumer (unary function from T to void), Predicate (unary function from T to boolean), and Supplier (nullary function to R).
  • Function shapes have a natural arity based on how they are most commonly used. The basic shapes can be modified by an arity prefix to indicate a different arity, such as BiFunction (binary function from T and U to R).
  • There are additional derived function shapes which extend the basic function shapes, including UnaryOperator (extends Function) and BinaryOperator (extends BiFunction).
  • Type parameters of functional interfaces can be specialized to primitives with additional type prefixes. To specialize the return type for a type that has both generic return type and generic arguments, we prefix ToXxx, as in ToIntFunction. Otherwise, type arguments are specialized left-to-right, as in DoubleConsumer or ObjIntConsumer. (The type prefix Obj is used to indicate that we don't want to specialize this parameter, but want to move on to the next parameter, as in ObjIntConsumer.) These schemes can be combined, as in IntToDoubleFunction.
  • If there are specialization prefixes for all arguments, the arity prefix may be left out (as in ObjIntConsumer).
Since:
1.8
See Also:
FunctionalInterface
  • Interface Summary 
    Interface Description
    BiConsumer<T,​U>
    Represents an operation that accepts two input arguments and returns no result.
    BiFunction<T,​U,​R>
    Represents a function that accepts two arguments and produces a result.
    BinaryOperator<T>
    Represents an operation upon two operands of the same type, producing a result of the same type as the operands.
    BiPredicate<T,​U>
    Represents a predicate (boolean-valued function) of two arguments.
    BooleanSupplier
    Represents a supplier of boolean-valued results.
    Consumer<T>
    Represents an operation that accepts a single input argument and returns no result.
    DoubleBinaryOperator
    Represents an operation upon two double-valued operands and producing a double-valued result.
    DoubleConsumer
    Represents an operation that accepts a single double-valued argument and returns no result.
    DoubleFunction<R>
    Represents a function that accepts a double-valued argument and produces a result.
    DoublePredicate
    Represents a predicate (boolean-valued function) of one double-valued argument.
    DoubleSupplier
    Represents a supplier of double-valued results.
    DoubleToIntFunction
    Represents a function that accepts a double-valued argument and produces an int-valued result.
    DoubleToLongFunction
    Represents a function that accepts a double-valued argument and produces a long-valued result.
    DoubleUnaryOperator
    Represents an operation on a single double-valued operand that produces a double-valued result.
    Function<T,​R>
    Represents a function that accepts one argument and produces a result.
    IntBinaryOperator
    Represents an operation upon two int-valued operands and producing an int-valued result.
    IntConsumer
    Represents an operation that accepts a single int-valued argument and returns no result.
    IntFunction<R>
    Represents a function that accepts an int-valued argument and produces a result.
    IntPredicate
    Represents a predicate (boolean-valued function) of one int-valued argument.
    IntSupplier
    Represents a supplier of int-valued results.
    IntToDoubleFunction
    Represents a function that accepts an int-valued argument and produces a double-valued result.
    IntToLongFunction
    Represents a function that accepts an int-valued argument and produces a long-valued result.
    IntUnaryOperator
    Represents an operation on a single int-valued operand that produces an int-valued result.
    LongBinaryOperator
    Represents an operation upon two long-valued operands and producing a long-valued result.
    LongConsumer
    Represents an operation that accepts a single long-valued argument and returns no result.
    LongFunction<R>
    Represents a function that accepts a long-valued argument and produces a result.
    LongPredicate
    Represents a predicate (boolean-valued function) of one long-valued argument.
    LongSupplier
    Represents a supplier of long-valued results.
    LongToDoubleFunction
    Represents a function that accepts a long-valued argument and produces a double-valued result.
    LongToIntFunction
    Represents a function that accepts a long-valued argument and produces an int-valued result.
    LongUnaryOperator
    Represents an operation on a single long-valued operand that produces a long-valued result.
    ObjDoubleConsumer<T>
    Represents an operation that accepts an object-valued and a double-valued argument, and returns no result.
    ObjIntConsumer<T>
    Represents an operation that accepts an object-valued and a int-valued argument, and returns no result.
    ObjLongConsumer<T>
    Represents an operation that accepts an object-valued and a long-valued argument, and returns no result.
    Predicate<T>
    Represents a predicate (boolean-valued function) of one argument.
    Supplier<T>
    Represents a supplier of results.
    ToDoubleBiFunction<T,​U>
    Represents a function that accepts two arguments and produces a double-valued result.
    ToDoubleFunction<T>
    Represents a function that produces a double-valued result.
    ToIntBiFunction<T,​U>
    Represents a function that accepts two arguments and produces an int-valued result.
    ToIntFunction<T>
    Represents a function that produces an int-valued result.
    ToLongBiFunction<T,​U>
    Represents a function that accepts two arguments and produces a long-valued result.
    ToLongFunction<T>
    Represents a function that produces a long-valued result.
    UnaryOperator<T>
    Represents an operation on a single operand that produces a result of the same type as its operand.